This invention relates to wellbore electric cables, as well as methods of manufacturing and using such cables. In one aspect, the invention relates to ruggedized optical fibers useful for wellbore slickline electric cables.
Generally, geologic formations within the earth that contain oil and/or petroleum gas have properties that may be linked with the ability of the formations to contain such products. For example, formations that contain oil or petroleum gas have higher electrical resistivity than those that contain water. Formations generally comprising sandstone or limestone may contain oil or petroleum gas. Formations generally comprising shale, which may also encapsulate oil-bearing formations, may have porosities much greater than that of sandstone or limestone, but, because the grain size of shale is very small, it may be very difficult to remove the oil or gas trapped therein. Accordingly, it may be desirable to measure various characteristics of the geologic formations adjacent to a well before completion to help in determining the location of an oil- and/or petroleum gas-bearing formation as well as the amount of oil and/or petroleum gas trapped within the formation.
Logging tools, which are generally long, pipe-shaped devices, may be lowered into the well to measure such characteristics at different depths along the well. These logging tools may include gamma-ray emitters/receivers, caliper devices, resistivity-measuring devices, neutron emitters/receivers, and the like, which are used to sense characteristics of the formations adjacent the well. A wireline cable, such as a slickline cable, connects the logging tool with one or more electrical power sources and data analysis equipment at the earth's surface, as well as providing structural support to the logging tools as they are lowered and raised through the well. Generally, the wireline slickline cable is spooled out of a drum unit from a truck or an offshore set up, over pulleys, and down into the well.
Wireline cables, or even permanent downhole monitoring cables, often include optical fibers for data transmittance. While optical fiber components in wireline or permanent monitoring cables have a vast potential for data transfer applications there are several weaknesses that make them vulnerable to damage in oilfield operations, including such weaknesses as: exposure to hydrogen at high temperatures results in a “darkening” of the optical fiber which reduction in data carrying capacity; limited linear stretch of the fiber as compared to the other cable components, thus requiring additional fiber length to be built in to the optical fiber components, which complicates the manufacturing process; volatilization of volatile organic compounds (VOCs) in coatings or other polymeric protective layers on the optical fibers releases additional hydrogen, which can attack and darken the fiber; hydrolytic attack of glass in the presence of water, which can lead to brittleness in the glass and susceptibility to data transmittance degradation; or lack of transverse toughness of the fiber component construction leads to potential point loading and micro-bending issues, which may lead to mechanical failure of the fiber and/or increased data attenuation.
The common approach used to create more rugged optical fiber components is to pultrude a long-fiber-reinforced epoxy thermoset resin jacket over a commercially obtained optical fiber, as illustrated in FIG. 1. As shown in FIG. 1, the optical fiber 102 has a long-fiber-reinforced epoxy thermoset resin jacket 104 pultruded thereupon to form the ruggedized optical fiber.
This approach to optical fiber ruggedizing has several disadvantages, including damage to optical fibers from point loading during the pultrusion process, shrinkage that occurs as the epoxy cures can impinge on the optical fiber and create signal attenuation problems, and handling the optical fibers more carefully to reduce the likelihood of point loading and overpull during the pultrusion process makes manufacturing difficult and time-consuming. A high incidence of signal attenuation encountered with these optical fiber components is unacceptable for use in oilfield DTS measurements, and often, the components may only be used for data transfer, and not as a conductor and data transfer device.
Thus, the need exists for wellbore electrical cables with ruggedized optical fibers, which remain durable during and after the pultrusion manufacturing process, while having conductor capability. Ruggedized optical fibers useful for forming cables which overcome one or more of the problems detailed above would be highly desirable, and the need is met at least in part by the following invention.